The cost of making the former engine shed site, off Churchfields Road, safe is not being revealed by the council, only that “the cost is included in a package of works that the council will be carrying out under the terms of the transfer of the land to the police, and this is reflected in the value of the land”.

The police will pay the council £600,000 for a long lease on the site.

The contamination is confined to the part of the site where the shed stood before it was demolished in the early 1970s.

The council said: “The majority of asbestos roofing tile material which formed the roof finish for the shed was deposited into the shed maintenance pits and backfilled with soil.”

It added that 725 cubic metres of contaminated soil would have to be removed.

Local councillors are hoping to see the land on either side of the unit used for commuter parking and a park.

A spokesman for Police Commissioner Angus Macpherson – who is recovering from a heart attack – has said previously that “work is currently taking place” on how much the new custody unit will cost to build.

Details are expected to be published once contracts have been signed.

Asked how much it would have cost to modernise the custody unit at Wilton Road to meet Home Office guidelines, the spokesman said: “It would have required demolition of the unit and a rebuild.

“A detailed costing exercise has not taken place, because this would have required costs to be incurred in drawing up plans and obtaining quotes.

“However, it is not unrealistic to suggest that the cost would have been £2m to £3m. This sum would not have dealt with all the other faults of this 1960s building or reduced the annual running cost liability.”

The Commissioner’s office has said previously that funds had been set aside to finance this work and that these will now be used, along with £2m from the sale of the police station to the council, to finance the new unit and a new police station at the Five Rivers community campus.

The old police station will be turned into a university technical college.

There will be a public meeting in Salisbury next month to display the proposals for the custody unit before they are submitted for planning permission.